Magnetic gear arrangement having a variable gear ratio

ABSTRACT

A magnetic gear arrangement is provided having a magnetically active gear member that generates a first magnetic field, which is modulated by interpoles. The modulated magnetic field generates magnetic poles on a magnetically passive gear member, and these poles form a second magnetic field. The material of the passive gear member is sufficiently magnetically hard that the first and second magnetic fields interact to couple the motion of the active and passive gear members according to a given gear ratio.

This is a continuation of U.S. application Ser. No. 13/060,844 filedFeb. 25, 2011, which is a National Phase Application ofPCT/EP2009/005384 filed Jul. 24, 2009. This application also claimspriority to GB 0817046.6 filed Sep. 18, 2008. The prior applications arehereby incorporated by reference herein in its entirety.

The present invention relates to magnetic gear arrangements,particularly magnetic gear arrangements having a variable gear ratio.

Gearboxes and gear arrangements are utilised in a wide range ofsituations in order to couple drive mechanisms. Traditionally, gearboxeshave been formed from gear wheels having appropriate teeth numbers andsizes to provide a desired gear ratio. However, such gearboxes have anumber of disadvantages. Firstly, they require the use of lubricatingoils, which may act as contaminants or fire hazards and may proveineffective in hot or cold environments, where the oil viscosity varies,or in a low pressure environment, where the oil may evaporate.Furthermore, gearboxes based on gear wheels may be noisy, making themunacceptable for low noise environments such as in hospitals, librariesand residential areas, or for use in clandestine military activity.

More recently, magnetic gearboxes have been provided which compriserespective gear rotors with interpoles between them. The rotorsincorporate permanent magnets, and the interpoles, or pole members orelements, act to modulate the magnetic flux transferred between the gearrotors. Such magnetic gearboxes enable a speed-changing mechanicaldevice to be provided in which there is no mechanical contact betweeninput and output shafts, thus avoiding many of the problems of noise andwear that arise in gearboxes having contacting moving parts.

FIG. 1 shows a schematic plan view of a typical magnetic geararrangement of the prior art. The magnetic gear arrangement 100 is anepicyclic gearbox and comprises an inner rotor 120 and an outer rotor160. Permanent magnets 140,180 are fixed to the inner and outer rotors120,160. The permanent magnets 140 affixed to the inner rotor 120 havealternating polarity along the circumference of the rotor. Similarly,the permanent magnets 180 affixed to the outer rotor 160 havealternating polarity along the circumference of that rotor. Typically,one rotor is mechanically coupled to a drive mechanism and the otherrotor is mechanically coupled to a driven mechanism.

The inner and outer rotors 120,160 have different numbers of permanentmagnets 140,180. Typically, the number of permanent magnets affixed tothe outer rotor 160 is greater than that affixed to the inner rotor 120.

Interpoles 200 are provided between the inner rotor 120 and the outerrotor 160 and form an array having a cylindrical shape.

The interpoles 200 modulate the magnetic field produced by the innerrotor 120 and the magnetic field produced by the outer rotor 160, so asto couple the two fields and hence the motion of the rotors. The numberof interpoles is a factor in determining the gear ratio of the magneticgearbox.

The motion of the rotors 120,160 may be either co-rotational orcounter-rotational, depending on the number of magnets affixed to eachrotor and the number of interpoles.

The present invention provides a magnetic gear arrangement in whichmagnetic poles are formed within a magnetically passive gear memberthrough the magnetic field generated by a magnetically active gearmember.

In particular, a first aspect of the invention may provide a magneticgear arrangement comprising:

a magnetically active gear member for generating a first magnetic field,

a magnetically passive gear member, and

interpoles between said magnetically active member and said magneticallypassive gear member for modulating the first magnetic field;

wherein magnetic poles are generated within the magnetically passivegear member by the modulated first magnetic field, the generated polesforming a second magnetic field; and

wherein the magnetically passive gear member is formed of sufficientlymagnetically hard material that the first magnetic field couples to thesecond magnetic field to produce a gear ratio between the magneticallyactive and the magnetically passive gear members.

The magnetic gear arrangement of the present invention includes amagnetically passive gear member that need not have permanent magnetsaffixed to it. Instead, magnetic poles are formed on this gear memberthrough the action of a modulated magnetic field generated elsewherewithin the arrangement. As a result, the number of magnetic poles on thegear member may be varied and the range of gear ratios attainable by themagnetic gear arrangement may be increased.

A further advantage of a magnetically passive gear member is that it mayhave a simple construction, since there is no requirement for permanentmagnets. As a result, the magnetically passive gear member will tend tobe cheaper than known magnetically active gear members and may exhibitbetter strength. Magnetically hard materials tend to have high tensilestrength, which contributes further to the mechanical properties of thepassive gear member. Typically, the magnetically active and magneticallypassive gear members are rotors. Strength and stability of a rotor areparticularly important at high rotational speeds and so in general, themagnetically passive gear member is configured to act as a high speedrotor relative to the magnetically active gear member.

The ability of the passive gear member to withstand high rotationalspeeds may make the magnetic gear arrangement of the first aspect of theinvention particularly suitable for specialist applications such asgyroscopes.

The magnetically hard material is characterised by high levels ofhysteresis and a high coercivity. In general, the coercivity is at least10 000 Amperes per meter. Typically, the magnetically hard material ischrome steel or cobalt steel.

The modulated first magnetic field may generate two distributions ofmagnetic poles on the magnetically passive gear member. The number ofpoles in the high-pole number first distribution corresponds to the sumof the number of interpoles and the number of poles in the firstmagnetic field. The number of poles in the low-pole number seconddistribution corresponds to the difference between these two numbers.

It is generally preferable that the number of poles formed on themagnetically passive gear member should correspond to the low-polesecond distribution of poles, since this allows the gear member tooperate as a high speed gear member. Therefore slots may be provided onthe magnetically passive gear member to disrupt the formation of amagnetic pole distribution corresponding to the high-pole firstdistribution.

In the case of a magnetically passive gear member that is a rotor, theseslots will typically extend radially inwards from the outer surface ofthe rotor.

It is thought that the high-pole distribution tends to be restricted tothe surface of the magnetically passive gear member, while the low-poledistribution tends to penetrate to the core of the member. Thus, byproviding slots at the surface whose spacing is incompatible with thespacing of the high-pole distribution, it is possible to selectivelyinhibit the formation of this pole distribution and therefore to promotethe formation of the low-pole distribution. The spacing of the slots maybe regular or irregular. The slots may have identical dimensions or mayhave different dimensions.

The slots provided on the magnetically passive gear member may be filledwith non-magnetic material rather than allowed to be open to the air.This may reduce windage losses, since it allows a smooth outer surfaceof the gear member to be provided.

Alternatively, the disrupting slots may contain an electrical windingfor producing various electromagnetic effects such as improved starting,and damping of unwanted oscillations. This winding may be a cage windingsuch as that used in a squirrel cage induction motor, or an amortisseurwinding used to damp transient behaviour in an electrical synchronousmachine. Possibly the slots may further contain electrically insulatingmaterial so that the electrical winding may be electrically insulatedfrom the magnetically passive gear member.

The magnetic gear arrangement of the present invention is typicallyconfigured so that the interpoles remain stationary when the gearbox isin operation. However, alternative configurations are possible, in whichthe interpoles move during use and one of the gear members (preferablythe magnetically active gear member) remains stationary.

The magnetically active gear member may comprise permanent magnets.

The magnetically active gear member may be a stator and the magneticallypassive gear member may be a rotor.

The magnetically active gear member may comprise electrical windingsthrough which an electrical current may be passed to generate the firstmagnetic field.

The magnetic gear arrangement may form a hysteresis motor or ahysteresis generator.

The magnetically active and magnetically passive gear members may beelongate to form a linear actuator.

The magnetically active gear member may be tubular and the magneticallypassive gear member may be arranged within the magnetically active gearmember.

The magnetic gear arrangement of the present invention may be adapted toprovide a hysteresis motor, by converting the magnetically active gearmember into a stator having electrical windings that may generate amagnetic field.

Thus, a second aspect of the invention may provide a hysteresis motorcomprising:

a stator having windings through which an electrical current may bepassed to generate a first magnetic field;

a rotor; and

interpoles between said stator and said rotor for modulating the firstmagnetic field;

wherein magnetic poles are generated within the rotor by the modulatedfirst magnetic field, the generated poles forming a second magneticfield; and

wherein the rotor is formed of sufficiently magnetically hard materialthat the first magnetic field couples to the second magnetic field toinduce rotational motion of the rotor at a rotational speed that isdependent on the number and/or the spacing of said interpoles. Thehysteresis motor of the second aspect of the invention may allow highrotor speeds to be generated from relatively low frequency electriccurrent, e.g. 50 Hz, 60 Hz, or 400 Hz. Electric current of thesefrequencies is widely available and easy to generate. The interpoleseffectively provide an in-built gearing mechanism, that is, they allowthe relationship between the speed of the rotor and the frequency of theelectric current to be varied e.g. by changing the number of interpolesand/or their spacing.

By applying an external mechanical drive to the hysteresis motor of thesecond aspect of the invention, the device may be used to generateelectrical power. The external mechanical drive causes the magnetic axisof the rotor to lead that of the stator, so as to generate electricalpower in the windings of the stator.

Thus, a third aspect of the invention may provide a hysteresis generatorcomprising:

a stator having windings through which an electrical current may bepassed to generate a first magnetic field;

a rotor; and

interpoles between said stator and said rotor for modulating the firstmagnetic field;

wherein magnetic poles are generated within the rotor by the modulatedfirst magnetic field, the generated poles forming a second magneticfield; and

wherein the rotor is formed of sufficiently magnetically hard materialthat when an external mechanical drive is used to induce rotationalmotion of the rotor, the second magnetic field also rotates, thusgenerating a voltage across the windings of the stator, the voltagefrequency being dependent on the number and/or the spacing of saidinterpoles.

Typically, the magnitude of the voltage generated by the hysteresisgenerator is also dependent on the number and/or the spacing of saidinterpoles.

Thus, the third aspect of the invention provides a hysteresis generatorhaving an in-built gearing mechanism. This gearing mechanism is providedby the interpoles of the generator and allows electrical power to beprovided at commercially useful frequencies (e.g. 50 Hz, 60 Hz, or 400Hz) from a rotor moving at a high rotational velocity. The mechanicaldrive for the rotor may be provided e.g. by a high-speed aircraftengine. In this case, the rotor preferably contains cobalt, which has ahigh Curie temperature and so retains magnetisation at the highoperating temperatures typically experienced in this application.

Both the hysteresis motor of the second aspect of the invention and thehysteresis generator of the third aspect of the invention may beconfigured such that the rotor has slots to disrupt the formation ofcertain magnetic pole distributions. Preferably the slots extendradially inwards from the outer surface of the rotor.

The magnetically hard material of the rotor is characterised by highlevels of hysteresis and a high coercivity. In general, the coercivityis at least 10 000 Amperes per meter. Typically, the magnetically hardmaterial is chrome steel or cobalt steel.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows a schematic plan view of a gearbox of the prior art.

FIG. 2 shows a schematic plan view of a gearbox of a first embodiment ofthe invention.

FIG. 3 shows a schematic plan view a gearbox of a second embodiment ofthe invention.

FIG. 4(a) shows a schematic section view of a gearbox of a thirdembodiment of the invention.

FIG. 4(b) shows schematic front views of the gear rotors of theembodiment shown in FIG. 4(a).

FIG. 5 shows a schematic plan view of a linear actuator according to thepresent invention.

FIG. 6 shows a schematic plan view of another linear actuator accordingto the present invention.

FIG. 7 shows a schematic section view of a tubular actuator according tothe present invention.

FIG. 2 shows a schematic plan view of a magnetic gear arrangement of thepresent invention. The magnetic gear arrangement 10 is an epicyclicgearbox and comprises a magnetically passive inner rotor 12 and amagnetically active outer rotor 16. Between the two rotors lies a set ofinterpoles 20, arranged to form a cylindrical array.

A plurality of permanent magnets 18 are affixed to the outer rotor 16,and have alternating polarity along the circumference of the rotor. Asan alternative to magnets, the outer rotor may have electrical windingsassociated with it for generating a magnetic field.

The interpoles 20 are for modulating the magnetic field generated by thepermanent magnets 18, and are made from a magnetically soft materiale.g. electrical steel. The interpoles are typically laminated, so as tominimise losses caused by eddy currents.

The inner rotor 12 comprises a cylinder of magnetically hard materiale.g. chrome steel or cobalt steel. This material typically has acoercivity of at least 10,000 Amperes per meter During operation of thegearbox, magnetic poles are formed on this cylinder by the magneticfield generated by the permanent magnets 18 on the outer rotor 16, themagnetic field being modulated by the interpole members 20.

Since the cylinder of the inner rotor 12 is made of magnetically hardmaterial, it exhibits a significant level of hysteresis. Thus, themagnetic poles formed on the cylinder have some stability and are slowto change in response to the movement of the outer rotor 16. As aresult, the inner rotor 12 will tend to rotate in response to rotationof the outer rotor 16.

At the same time, however, it is possible to induce different numbers ofmagnetic poles on the cylinder of the inner rotor 12, depending on thenumber of permanent magnets 18 on the outer rotor 16, and on the numberof interpoles 20. Thus, the configuration of the inner rotor 12 allows agearbox to be provided in which the gear ratio may be varied.

Typically, in this magnetic gear arrangement, the number of magneticpoles generated on the cylinder of the inner rotor 12 is determined byone of two possible configurations. The first configuration has a numberof magnetic poles corresponding to the sum of the number of permanentmagnets affixed to the outer rotor 16 and the number of interpoles 20,and therefore has a high number of magnetic poles. The secondconfiguration has a number of magnetic poles corresponding to thedifference between the number of permanent magnets affixed to the outerrotor 16 and the number of interpole members 20, and therefore has a lownumber of magnetic poles, i.e. a lower number of magnetic poles than thefirst configuration.

It is desirable that the inner rotor 12 should have a higher rotationalspeed than the outer rotor 16, since the inner rotor 12 is better ableto withstand the mechanical stresses induced by rotational motion. Thisis because the magnetic material of the inner rotor 12 is in the form ofa cylinder, whereas the outer rotor 16 has separately-formed permanentmagnets affixed to it, which may become detached at high rotationalspeeds.

Thus, it is desirable that a low number of magnetic poles should beformed on the cylinder of the inner rotor 12 i.e. a number of polescorresponding to the second configuration of magnetic poles.

In order to achieve this, the cylinder of the inner rotor 12 hasdisrupting slots 14, which extend radially inwards from the outersurface of the cylinder. These slots tend to disrupt the formation ofmagnetic pole distributions having a high number of magnetic poles, ifthe spacing of the slots does not match the spacing of the poles. Theformation of pole distributions having a low number of poles isdisrupted to a much lesser extent. This is considered to be due to thefact that higher polarity magnetic fields will tend to be confined tothe radially outer regions of the cylinder of the inner rotor 12, whilelower polarity magnetic fields tend to permeate the radially innerregions of the cylinder.

The slots 14 may be filled with a non-magnetic material. This is thoughtto reduce windage losses. Alternatively, the disrupting slots maycontain an electrical winding for producing various electromagneticeffects such as improved starting, and damping of unwanted oscillations.The electrical winding may form part of a cage winding, such as thatused in a squirrel cage induction motor.

The gear arrangement of FIG. 2 may be converted to a motor. In thiscase, the magnetically passive gear member (equivalent to the innerrotor 12) is configured to rotate, while the magnetically active gearmember (equivalent to the outer rotor 16) is converted to a stator.

The stator is provided with electrical windings, rather than permanentmagnets, and these windings may allow the magnetic gear arrangement tobehave as a motor. For example, if the windings are wound in wayssimilar to known electrical machines and electrical power is supplied tothe windings, the magnetic gear arrangement will behave as a hysteresismotor with the inner rotor 12 producing a mechanical power output.

An advantage of a hysteresis motor configured in this way is that lowfrequency electric current may be used to generate high motor speeds.The interpoles 20 effectively provide an in-built gearing mechanism forproducing high motor speeds from low frequency electric current. FIG. 3shows a schematic plan view of an alternative configuration of themagnetic gear arrangement of the present invention. The gear arrangement30 is an epicyclic gearbox and has a magnetically active inner rotor 32,to which a plurality of permanent magnets 34 is affixed. Themagnetically passive outer rotor 36 comprises a cylinder of magneticallyhard material on which a plurality of magnetic poles are formed inresponse to the magnetic field generated by the permanent magnets 34 ofthe inner rotor 32.

The magnetic field generated by the permanent magnets 34 of the innerrotor 32 is modulated by interpoles 40 located between the inner andouter rotors 32,36.

Slots 38 in the outer rotor 36 tend to restrict the formation ofmagnetic pole distributions having a high number of poles. To reducewindage losses, the slots 38 may be filled with a non-magnetic material.The slots 38 may also carry an electrical winding such as a cage windingas used in a squirrel cage induction motor, or an amortisseur winding asused in an electrical synchronous machine. The slots 38 may also containelectrical insulation so that the electrical winding is insulated fromthe body of the inner rotor.

In this magnetic gear arrangement, the outer rotor 36 is configured torotate at a higher speed than the inner rotor 32.

FIG. 4 shows a magnetic gear arrangement of another embodiment of thepresent invention. FIG. 4(a) shows a side view of the gear arrangement,while FIG. 4(b) shows two views taken from the front and rear of thegear arrangement. The gear arrangement 50 is a co-axial gearbox having amagnetically active first rotor 52 to which a plurality of permanentmagnets 54 is affixed. The magnetically passive second rotor 56comprises a cylinder of magnetically hard material on which a pluralityof magnetic poles are formed in response to the magnetic field generatedby the permanent magnets 54 of the first rotor 52. The central axes ofthe first and second rotors 52,56 coincide.

The magnetic field generated by the permanent magnets 54 of the firstrotor 52 is modulated by interpoles 60 that are arranged to form acylindrical array. The first and second rotors 52,56 are located withinthis cylindrical array, each at a respective end of the array.Alternatively, the first and second rotors 52,56 may lie outside thecylindrical array of interpoles 60, each rotor being located at arespective end of the array.

Slots 58 in the second rotor 56 tend to restrict the formation ofmagnetic pole arrangements having a high number of poles.

In this magnetic gear arrangement, the second rotor 56 is configured torotate at a higher speed than the first rotor 52.

In an alternative configuration of this embodiment, the central axes ofthe first and second rotors may be displaced, rather than coincident. Inthis case, the array of interpoles will have the specific shape requiredto extend between the first and second rotors. Such an arrangement ofinterpoles is described, for example, in WO2007/135360.

FIG. 5 shows a schematic plan view of a linear actuator according to thepresent invention. The actuator 70 comprises an elongate magneticallyactive first member 72 to which a plurality of permanent magnets 74 areaffixed. The magnetically passive second member 76 comprises an elongateportion of magnetically hard material that is aligned with the firstelongate member 72 and on which a plurality of magnetic poles are formedin response to the magnetic field generated by the permanent magnets 74of the first member 72.

The magnetic field generated by the permanent magnets 74 of the firstmember 72 is modulated by interpoles 80 located between the first andsecond members 72,76.

Slots 78 provided along the length of the second member 76 tend torestrict the formation of magnetic pole distributions having a highnumber of poles. Thus the formation of pole distributions having a lownumber of poles is promoted.

In this actuator, the first member 72 is configured as a low-speedmember and the second member 76 is configured as a high-speed member.

FIG. 6 shows a schematic plan view of another linear actuator accordingto the present invention. The actuator 90 is a double-sided version ofthe linear actuator shown in FIG. 5, the first member 92, the interpolemembers 99 and the second member 96 being arranged so that the actuatoris symmetrical about the second member 96.

FIG. 7 shows a schematic plan view of a tubular actuator according tothe present invention. The actuator 110 has a magnetically activetubular first member 112 to which permanent magnets 114 are affixed. Themagnetically passive second member 116 comprises an elongate portion ofmagnetically hard material that is aligned with the central axis of thetubular first member 112 and on which a plurality of magnetic poles areformed in response to the magnetic field generated by the permanentmagnets 114 of the first member 112.

The magnetic field generated by the permanent magnets 114 of the firstmember 112 is modulated by interpoles 119 located between the first andsecond members 112,116.

Slots 118 are provided along the length of the second member 116, eachslot extending around the second member 116 in a circumferentialdirection. These slots 118 promote the formation of magnetic poledistributions having a low number of poles.

In the tubular actuator, the first member 112 is configured as alow-speed member and the second member 116 is configured as a high-speedmember.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

The invention claimed is:
 1. A magnetic gear arrangement comprising: a magnetically active gear member for generating a first magnetic field, a magnetically passive gear member, and a number of interpoles between said magnetically active gear member and said magnetically passive gear member for modulating the first magnetic field, wherein: a number of magnetic poles are generated within the magnetically passive gear member by the modulated first magnetic field, the generated poles forming a second magnetic field, the magnetically passive gear member is formed of sufficiently magnetically hard material that the first magnetic field couples to the second magnetic field to produce a gear ratio between the magnetically active and the magnetically passive gear members, and a magnetically active gear member has a plurality of poles, the number of generated magnetic poles is equal to a sum of the number of poles on the magnetically active gear member and the number of interpoles.
 2. A magnetic gear arrangement according to claim 1, wherein the magnetically hard material has a coercivity of at least 10,000 Amperes per meter.
 3. A magnetic gear arrangement according to claim 1, wherein said magnetically passive gear member has slots which disrupt the generation of selected distributions of magnetic poles.
 4. A magnetic gear arrangement as claimed in claim 3 wherein a non-magnetic material is provided in the slots.
 5. A magnetic gear arrangement as claimed in claim 3 wherein the slots contain an electrical winding.
 6. A magnetic gear arrangement according to claim 1, wherein said magnetically active and magnetically passive gear members are rotors.
 7. A magnetic gear arrangement according to claim 6, wherein said magnetically passive gear member is configured to act as a high speed rotor relative to said magnetically active gear member.
 8. A magnetic gear arrangement as claimed in claim 1 wherein said magnetically active gear member comprises permanent magnets.
 9. A magnetic gear arrangement as claimed in claim 1 wherein said magnetically active and magnetically passive gear members are elongate to form a linear actuator.
 10. A magnetic gear arrangement as claimed in claim 9 wherein said magnetically active gear member is tubular and said magnetically passive gear member is arrangement within the magnetically active gear member.
 11. A magnetic gear arrangement as claimed in claim 1 wherein said magnetically active gear member comprises permanent magnets.
 12. A magnetic gear arrangement as claimed in claim 8 wherein said magnetically active gear member comprises electrical windings through which an electrical current may be passed to generate the first magnetic field.
 13. A magnetic gear arrangement as claimed in claim 12 wherein the magnetic gear arrangement forms a hysteresis motor or a hysteresis generator.
 14. A hysteresis motor comprising: a magnetically active gear member comprising a stator having windings through which an electrical current may be passed to generate a first magnetic field; a rotor; and a number of interpoles between said stator and said rotor for modulating the first magnetic field, wherein: a number of magnetic poles are generated within the rotor by the modulated first magnetic field, the generated poles forming a second magnetic field, the rotor is formed of sufficiently magnetically hard material that the first magnetic field couples to the second magnetic field to induce rotational motion of the rotor at a rotational speed that is dependent on the number and/or the spacing of said interpoles, and the magnetically active gear member has a plurality of poles, a number of generated magnetic poles is equal to a sum of the number of poles on the magnetically active gear member and the number of interpoles.
 15. A hysteresis generator comprising: a magnetically active gear member comprising a stator having windings through which an electrical current may be passed to generate a first magnetic field; a rotor; and a number of interpoles between said stator and said rotor for modulating the first magnetic field, wherein: a number of magnetic poles are generated within the rotor by the modulated first magnetic field, the generated poles forming a second magnetic field, the magnetically active gear member has a plurality of poles, the rotor is formed of sufficiently magnetically hard material that when an external mechanical drive is used to induce rotational motion of the rotor, the second magnetic field also rotates, thus generating a voltage across the windings of the stator, the voltage frequency being dependent on the number and/or the spacing of said interpoles, and the number of generated magnetic poles is equal to a sum of the number of poles on the magnetically active gear member and the number of interpoles. 